Method of making thin walled plated pistons for internal combustion engines



Patented Dec. 5, 1939 UNITED STATES METHOD OF MAKING THIN W \LLED PLATED PISTONS FOR INTERNAL COMBUSTION ENGINES Russell H. McCarroll, Dearborn, Mich., assignor to Ford Motor Company, Dear-born, MiClL, a corporation of Delaware No Drawing. Application April 3, 1937,

, Serial No. 134,735

4 Claims.

This invention relates to light weight pistons made of ferrous metals and has for its object to provide a new and improved steel alloy especially adapted for constructing pistons for internal combustion engines which pistons will have many of the advantages of aluminum pistons without several of the disadvantages inherent in such aluminum pistons.

The present application is a continuation in 10 part of my application, Serial No. 4,126, filed January 30, 1935.

Aluminum pistons are advantageous in that the metal aluminum is an exceptionally good heat conductor so that the transfer of heat from one part of the piston to another part is rapid enough to allow the piston to remain cool even though comparatively high compression pressures are used. Furthermore, aluminum being very light in weight minimizes the reciprocating strains on the wrist pin and connecting rod bearings, and also requires a minimumpf counterweighting in the crankshaft to produce a running balance. These advantages are sufliciently important to make an aluminum piston prefer- 25 able to the ordinary cast iron piston for high speed engine use. However, they are obtained only with certain accompanying inherent disadvantages. These disadvantages are that alummum has a high coefi'icient of expansion so that compensating slots, Invar struts, or some other means must be provided to prevent piston slap during the warming up period of the motor. Furthermore, aluminum has a comparatively high coefiicient of friction with cast iron so that excessive piston weanresults, and further the cylinder surface attains a somewhat hotter temperature than when cast iron pistons are used, due to the increased friction.

Perhaps the greatest disadvantage inherent in 40 aluminum pistons is that the metal is comparat1vely soft so that thepiston ring grooves pound out or increase in width appreciably after only a few hundred hours of use. Furthermore, wear on the piston skirt causes the piston 4 to wobble as it reciprocates, which motion wears oil? the sharp edges of the ring grooves. These two conditions cause the engine to pump considerable oil. One of the reasons for oil pass-. ing by the pistons in internal combustion engines is that excessive clearance between the ring grooves and the rings, allows the oil upon the down stroke of the piston to be forced into the spacein back of the piston rings so that upon the succeeding down stroke of the piston this oil is 55 deposited on the cylinder walls above the ring. To prevent oil pumping, it is essential that there be a minimum clearance between the ring grooves and the piston rings.

The principal object of the present invention, as above pointed out, is to provide a steel alloy capable of being cast with exceptionally thin 5 walls so as to obtain a piston that would not weigh appreciably more than an aluminum piston, it being essential in the modern internal combustion automobile engine to keep the weight of the pistons to a minimum because of the very high speeds at which such engines are adapted to run. The making of a very thin wall casting has presented in the past almost insurmountable difficulties, not only from the standpoint of having a metal of sufficient fluidity-when in the molten state to flow freely into the narrow spaces in the mold, but also because of the great difficulty in the prevention of blow holes and other defects in the castings which resulted in such a large proportion of rejects or scrap as to render the use of exceptionally thin wall castings impractical for large production.

Another object of the present invention is to provide an alloy which in addition to having the requisite free flowing properties for molding, 5 would also be readily machinable.

Still another object of the invention is to provide a piston having the desired long wearing properties of the ferrous metals but with a higher carbon content than the average steels and to provide a form of heat treatment for the alloy Whichwould cause the excess of carbon above that which would readily go into solution with the iron to be formed throughout the matrix as small nodules of temper carbon uniformly distributed throughout the entire matrix.

It is still another object of the present invention to provide a piston formed of a ferrous alloy which when cast and heat treated, would re sult in the production of a matrix having the highly advantageous properties of a high grade steel. It has for some time been the practice in the automotive industry to employ cast iron pistons which have been plated with various metals, such as copper, lead and tin, to minimize the danger of the piston seizing or scoring during the breaking in or initial period of operation of the engine. Although it has been suggested that cast iron pistons might be plated with other metals, such as silver and cadmium, the use of cadmium as a plating for cast iron pistons has not, as far as I am aware, been entirely successful due .to the great difliculty in procuring a satisfactory adherent deposit upon the cast iron which metal, as is well known, is not as readily plated as steel.

It is, therefore, another object of the present invention to provide a piston alloy which will possess the property of being readily plated with an adherent deposit or plate of a metal having 5 better wearing properties than the copper, lead or tin heretofore used for the plating of cast iron pistons, and which also will have a higher fusion point to withstand the high temperatures encountered in internal combustion engines, than the tin plating which has heretofore been the most commonly used.

The above and other objects of the present invention will appear more fully from the following more detailed description.

Although a relatively wide variation in the limits of the constituents of my improved alloy may be resorted to within the following limits:

Per cent Carbon 1.00 to 2.00 Manganese .50 to 1.50 Silicon .50 to 1.50 Copper 1.50 to 3.50 Phosphorus maximum .12 26 Sulphur do .16 Iron Balance extended manufacture of pistons in accordance with the present invention has shown that the best results are obtained with an alloy falling 30 within the more narrow limits of the following analysis:

Per cent Carbon 1.40 to 1.60 Manganese .60 to 1.00 Silicon .90 to 1.30 Chromium .08 to .15 Phosphorus maximum .10 Sulphur do .08 Copper 2.50 to 3.00 40 Iron Balance The above alloy is cast by improved sand castmg methods. After the removal from the molds, the castings are then heated to approximately 1650 F. for twenty minutes, and then air cooled for thirty minutes to a maximum of 1200 F. The castings are then re-heated to 1400 F. and held at this temperature for approximately one hour, after which they are then slowly cooled to 1000? F., this last mentioned cooling taking approximately one hour to accomplish, after which the castings are air cooled to room temperature. The above mentioned heat treatment differs from the conventional normalizing heat treatment, the first step of the treatment being for the purpose of breaking up the grains and starting a secondary graphitization and also to insure complete saturization of the matrix to form pearlite, the heat treatment being stopped after the castings have been at the critical point long enough to insure the above results but before there is any appreciable formation of flake graphite. The second slow re-heating and holding the temperature at the lower 1400 F. point insures complete graphitization of the excess carbon and the collection of such excess into small nodules of temper carbon and also at the same time results in spheroidizing the pearlite, while the last slow cooling prevents embrittlement of the metal and. the production of a hard and unchilled casting.

conductivity to such an extent that the crosssectional area or thickness of the piston head may be reduced to substantially that required to withstand the loads placed upon the piston head The resultant matrix is pearlitic with the copper.

7 during operation of the engine.

.a compression ratio of approximately 6.5 to 1.

When an ordinary cast iron piston is used, the piston head must be several times this thickness to give the necessary structural strength.

A further very marked advantage of pistons constructed with the alloy of the present invention is that due to the presence of the excess graphite in the form of temper carbon and also to the copper in solution in the matrix, the coefiicient of friction is considerably less than between cast iron and cast iron, or between aluminum and cast iron. Consequently less wear results on the piston walls and cylinders and less heat is developed, thereby reducing the tendency to destroy the essential oil film between the piston and cylinder.

As it has always been a problem in the automotive industry to insure the presence of the essential oil film between the piston and cylinder when the engine is new, and during what is known as the running in period of the engine, it has been customary to run an automobile for the first several hundred miles at very slow speeds. Therefore the plating of cast iron pistons with tin has been resorted to to overcome, to a certain extent, this difficulty. Tin, however, is subject to embrittlement when subjected to the high operating temperatures of the present high speed, high compression engine, and moreover, its low melting point does not enable it to withstand such temperatures. With a piston of the present invention, the matrix of which has all of the desirable properties of high grade steel insofar as taking a uniform highly adherent deposit or coating of any metals by electro-plating methods, it has been found highly desirable to plate the pistons of the present invention with a cadmium plate to overcome the difficulties heretofore encountered in the automotive industry in breaking in, or running in, the engine during the first few hundred miles of operation of the automobile. Cadmium plate is much superior to tin also because of the much greater hardness and durability of cadmium as compared with tin.

After the castings have been cast and heat treated, as above described, the pistons are machined to proper dimensions and are then cleaned in an electrolytic cleaner containing a solution of soda ash, trisodium phosphate and caustic potash. They are kept submerged in this solution which is maintained at a temperature of 200 to 210 F. of a current density of 50 to 60 amperes per square foot, 12 volts. An iron tank is used as an anode, and the period of submersion varies from 1 to 15 minutes, according to the condition of the work. The castings are then rinsed under cold running water and are next acid dipped in commercial muriatic acid;'then subjected to another cold water rinse; next an alkali dip in 10 percent solution of soda ash; then another cold water rinse, after which they are placed in an electrolytic bath for the cadmium plating operation. The bath consists of 8 ounces of sodium cyanide, 3 ounces cadmium oxide, 3

ounces caustic soda and 1% ounces corn syrup to one gallon of water. A current density of 20 to 25 amperes per square foot, 6-volts, and a temperature of 100 to 110 F. is maintained in the plating bath. 99 percent cadmium anodes are used and the time period of plating varies from 10 to 20 minutes, according to the articles to be plated. After removal from the plating bath, the pistons are subjected to a cold water rinse with running water and then are thoroughly cleaned with hotwater.

Pistons constructed in accordance with the present invention may be placed in the engines and automobiles may be immediately operated at full maximum speeds without any detrimental effects on such pistons when the automobiles are brand new.

No matter how smooth a surface may be secured upon the wearing surface of the piston by ordinary machining operations, or even by grinding, the metal seems to have a sufficiently pitted surface as to present a grave source of danger during the initial period of operation and until the surface is worn smooth. The use of cadmium plating entirely eliminates this source of danger, as the plate fills up the very small interstices on the surface of the metal, and presents a smooth surface. Due to the fact that the cadmium is much softer than the alloy of the piston, it very quickly is forced into the interstices in the surface of the metal almost immediately the engine has been started in operation, and as the cadmium is fairly resistant to Wear, the protective coating formed by the cadmium lasts until the harder metal of the alloy has assumed a very smooth, excellent wearing surface.

The alloy of the present invention has most exceptional properties with respect to its fluidity in the molten state and to form extremely thin wall castings. Over fifty thousand pistons per day are being cast in the'plant of applicants assignee in which the walls of the skirt portion of the piston as cast, have but a thickness of 0.090 to .130, or approximately 3 to A; of an inch, the skirt portion being 1% inches in length. When the pistons are machined, the thickness of the skirt wall is but from .030 to .035 of an inch, namely' Per cent Carbon 1.00 to 2.00 Manganese .50 to 1.50 Silicon .50 to 1.50 Copper 1.50 to 3.50 Phosphorus maximum .12 Sulphur do .16 Iron Balance then heating said casting to 1650 F. for twenty minutes, then air cooling said casting in thirty' minutes to 1200" F., then reheating said casting to 1400 F. and holding the casting at this temperature for one hour, then gradually cooling said casting to 1000 F., this last mentioned cooling taking one hour to accomplish, then air cooling said casting to room temperature, and then machining said casting to finished dimensions.

2. The method of forming an internal combustion engine piston consisting of forming a casting of the following compositions:

Per cent Carbon 1.00 to 2.00 Manganese .50to 1.50 Silicon .50 to 1.50 Copper 1.50 to 3.50 Phosphorus "maximum" .12 Sulphur do 16; Iron Balance then heating said casting to approximately 1650" F. for twenty minutes so as to break up the grains and start secondary graphitization, then air cooling the casting in thirty minutes to a maximum -of 1200 then reheating said casting to 1400 F. and holding same at this temperature for one hour to thereby complete the aforementioned graphitization while at the same time spheroidiz ing the pearlite, then slowly cooling said casting to 1000 F., this last mentioned cooling taking one hour to accomplish, then air cooling the casting to room temperature, and then machining said casting to finished dimensions.

3. The method of forming a thin walled inter- Diamond Cam Diamond turned Steel piston ground turned aluminum aluminum aluminum .001

undersize Frictional heat gain not under power .B. t. u. 56, 700 64, 500 63, 300 56, 800

Heat gain under power 65 H. P. at 3000 R. I. M.

B. t. u. 298, 100 332, 600 320, 670 309, 000 011 temp. not under power. 165 169 175 165 Oil temp. under power. 206 210 203 Fuel 65 H. P. at 3000 R. 38. l 40. 3 40. 7 3 Horse power (main) 83.4 36 75.7 3600 76.6 3000 81.1 3600 Compression (mam) 136.8 2000 138.1 2000 139.7 2000 Fuel average full throttle lbs. 11. P. hr.. .631 656 669 635 nal combustion engine piston consisting of forming a sand casting of the following composition:

then heating said casting to 1650 F. for twenty minutes, than air cooling said casting in thirty minutes to 1200 F., then reheating said casting to 1400 F. and holding the casting at this temperature for one hour, then gradually cooling said casting to 1000 F., this last mentioned cooling taking one hour to accomplish, then air cooling said casting to room temperature, then machining said casting to finished dimensions and providing said piston with an electro-plate coating of cadmium.

4. The method of forming an internal combustion engine piston consisting of forming a casting of the following compositions:

Per cent Carbon 1.40 to 1.60 Manganese .60 to 1.00 5 Silicon .90 to 1.30 Phosphorus maximum .10 Sulphur 1 do .08

' Copper 2.50 to 3.00

Iron Balance 10 then heating said casting to approximately 1650 F. for twenty minutes so as to break up the grains and start secondary graphitization, then air cooling the casting in thirty minutes to a 1 maximum of 1200 F., then reheating said casting to 1400 F. and holding same at this temperature for one hour to thereby complete the aforementioned graphitization while at the same time spheroidizing the pearlite, then slowly cooling 2(} said casting to 1000 F., this last mentioned cooling taking one hour to accomplish, then air cooling the casting to room temperature, and then machining said casting to finished dimensions. RUSSELL H. McCARROLL. 

